United States patent application publication 2010/0207589 discloses a method for operating a double-fed asynchronous generator, in which the stator is connected to a power supply grid. The rotor is operated via the drive train of the wind turbine. The rotor is electrically coupled to the power supply grid via a machine-side and a grid-side converter. In the event of the identification of short-circuit currents in the power supply grid, the present actual value of the machine-side rotor current is detected, frozen and used instead of the selected setpoints for the control of the machine-side rotor current.
U.S. Pat. No. 7,966,103 discloses a wind turbine and a method for operating the wind turbine in the event of grid faults with a low voltage. The known method envisages controlling the output current depending on a power stored in the system during a grid fault with a low voltage. The blade pitch angle is kept constant during the voltage drop.
U.S. Pat. No. 7,851,934 discloses a method for the control of a wind turbine, in which one or more rotor blades are driven in a fault mode in the event of a grid fault in order to keep the rotor speed stable within a predetermined range.
U.S. Pat. No. 7,834,472 discloses a method for the control of a wind turbine during faulty operation in the electrical power supply grid, in which one or more rotor blades of the wind turbine are driven during the grid fault in order to keep the temperature in the stator and/or rotor of the generator below a predetermined maximum temperature.
U.S. Pat. No. 7,714,458 discloses a method for operating a wind turbine, in which a blade pitch angle for the rotor blade is set depending on a rotational speed. By providing a new blade pitch angle, the wind turbine can be set to a new stable operating point without being isolated from the power supply grid for rapid load reduction.
U.S. Pat. No. 6,921,985 B2 discloses a method for the control of a wind turbine, in which a grid fault is identified and control of the blade pitch angle is performed in response to the grid fault, wherein the adjustment system of the blade pitch angle is fed by an uninterruptable power supply during the grid fault.
In general, the situation in the case of a wind turbine is such that the wind turbine cannot output the maximum power in the event of unexpected grid faults. The resultant quick dip in power of the wind turbine first results in an increase in rotational speed and, once the grid fault has come to an end, in a temporary rotational speed undershoot. In the event of a grid fault, the grid voltage decreases for a time of several seconds to such an extent that the output electrical active power of the wind turbine also reduces correspondingly, under some circumstances down to 0 kW. The reduced load on the drive train results in an increase in rotational speed. The rotational speed controller then compensates for the increased speed in the case of conventional wind turbines. As a result of the control loop dynamics, a rotational speed undershoot can then occur. The initially increased rotational speed subjects the wind turbine to a load and can also be problematic for a converter of the wind turbine.
In the case of grid faults with voltage dips, the power drop has the effect that the wind turbine can only output a reduced power to the grid and stores the excess power, by virtue of an increase in the rotational speed, in the rotor of the wind turbine. After the grid fault, a higher power is then demanded by the power controller, owing to the higher rotational speed, than is actually present in the wind. The rotational speed thus decreases again. The energy stored in the rotor is output to the grid within a short period of time which is not desirable from the point of view of the grid operators.
In the event of grid faults during nominal load operation of the wind turbine, said wind turbine operates at the nominal speed and control of the rotational speed and the power is performed by virtue of adjusting the blade pitch angle. It is not possible for any energy to be stored in the rotor since a control for the blade pitch angle causes the rotational speed of the rotor to return to the nominal rotational speed. After the grid fault has ended, when the output power is stepped up again, the rotational speed experiences a dip to values below the nominal rotational speed. The power controller then reduces the setpoint preset for the power in accordance with its rotational torque/speed curve in order to transfer the installation to a stable operating point. At the same time, the blade pitch controller responds and attempts to bring the rotational speed back to the nominal rotational speed by means of adjusting the blade pitch angle. Once the grid fault has been cleared, therefore, there is a power dip for a short period of time which should be avoided in respect of the stability of the electrical power supply grid.